1. Field of the Invention
This invention relates to lithium batteries. More particularly, the invention relates to lithium batteries that have a liquid electrolyte, but enjoy the mechanical advantages of a solid phase electrolyte without sacrificing the electrochemical advantages of a liquid electrolyte.
2. Description of the Prior Art
The advantages and construction of lithium batteries are well known. See, for example, Tsutsumi et al. U.S. Pat. No. 5,998,065, and Kim et al. U.S. Pat. No. 6,001,509. Lithium batteries serve well as secondary batteries, and are capable of being reduced in size as compared with present battery designs that are widely used. They are also capable of being reduced in weight so that in the areas, for example, of form factor, size, weight, safety and capacity, lithium batteries substantially exceed the capabilities of the present designs. Difficulties have been encountered, however, in devising satisfactory electrolyte-separator combinations that would maximize the potential available in the lithium battery technology.
Previously, both liquid or gel organic electrolyte and solid phase electrolyte lithium batteries have been proposed. The liquid electrolyte based designs previously suffered from substantial problems of safety and utility due to the inherent nature of the liquid phase electrolyte. It leaked out of the container if the container was ruptured. This destroyed the utility of the battery and risked, for example, fire, explosion, toxic release, and damage to expensive equipment. In general, the previous liquid electrolyte based lithium battery designs were fragile. The previous liquid based designs did, however, enjoy the advantage that they could be put through many charge-discharge cycles without significant loss of function. Also, prior liquid electrolyte based lithium batteries could be charged and discharged rapidly without undue heat build up, because the resistance of the liquid electrolyte was low. By contrast, prior solid electrolyte based lithium battery designs were rugged, but suffered from excessive heat build up during rapid charging and discharging, and were only good for a limited number of charge-discharge cycles.
It had previously been proposed to modify the surface energy of a reticulated polystyrene foam separator in a lithium battery so as to promote the retention of a liquid electrolyte within the pores of the separator. Such previous proposals involved, for example, incorporating a molecule in the polymer chain of the solid phase polystyrene that would increase its surface energy. Sulfonate containing molecules had been proposed for this purpose. The inclusion of sulfonate or other surface energy modifying molecules within the skeleton of the solid phase polymer caused an undesired negative impact on the physical properties of polystyrene. Alternatively, previous proposed expedients for increasing the surface energy of the porous separators in lithium batteries often required that the electrolyte be prepared separately from the skeleton so that the walls of the skeleton could be washed with a surfactant before the liquid electrolyte was added. This contributed undesirably to the cost and complexity of the manufacturing procedure. Also, the retention of the liquid electrolyte within the foam skeleton was less than unsatisfactory.
Itho U.S. Pat. No. 4,849,311 discloses an ionic conductor, liquid or solid, that is immobilized in the pores of a porous solid polymer membrane. Immobilization of the liquid ionic conductor is said to be accomplished by the combination of using pore sizes of less than 0.1 microns, an appropriate choice of solvent, and surface treatment. The disclosed surface treatments to control the wetability of the polymer membrane are plasma and graft polymerization on the surface. The liquid contact angle to the polymer is said to be not more than 90, and preferably not more than 70 degrees. Neither the use of nor the need for a solid surfactant at the interface between the solid and liquid phases is suggested.
The inclusion of solid surfactants in foamed thermoplastic materials is known. See, for example, Cobbs et al. U.S. Pat. No. 4,156,754. According to The disclosure of Cobb et al. a finely divided surfactant is mixed with a gas containing molten thermoplastic to stabilize the gas.
Those concerned with these problems recognize the need for an improved lithium battery.